Low levels of vitamin D may help explain the spread of colds and flu during the winter, according to a major international research project published today.
The researchers say their findings suggest that vitamin D supplements can protect against acute respiratory infections – and call for vitamin D fortification of food in affected regions.
The findings suggest that the supplements work best in those with the lowest levels of the vitamin – as might be found among people who stay indoors during the winter.
The findings, reported in The BMJ, come from an analysis of 25 randomised clinical trials involving 11,000 people in countries including the USA, UK, India, Afghanistan, Belgium and Italy.
This showed a 12% reduction in people succumbing to infection after taking vitamin D supplementation.
The researchers said the individual trials showed conflicting results, partly because of different levels of the vitamin the population.
Researcher Professor Adrian Martineau, from Queen Mary, University of London, UK, said: “This major collaborative research effort has yielded the first definitive evidence that vitamin D really does protect against respiratory infections.
“The bottom line is that the protective effects of vitamin D supplementation are strongest in those who have the lowest vitamin D levels, and when supplementation is given daily or weekly rather than in more widely spaced doses.
“Vitamin D fortification of foods provides a steady, low-level intake of vitamin D that has virtually eliminated profound vitamin D deficiency in several countries."
He added: "By demonstrating this new benefit of vitamin D, our study strengthens the case for introducing food fortification to improve vitamin D levels in countries such as the UK where profound vitamin D deficiency is common.”
The findings are challenged in the journal by Mark Bolland from the University of Auckland, New Zealand, and Alison Avenell from the University of Aberdeen, who say that the results may not apply to the general population.
Vitamin D supplementation to prevent acute respiratory infections: systematic review and meta-analysis of individual participant data’. Martineau et al. BMJ 16 February 2017 [abstract]
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